With the constantly widened applications in different fields thereof, cooling fans have been used with many electronic devices with special purposes, such as central processing units (CPU), servers, power supplies, communication chassis, and telecommunication base stations, to work in increasingly severe environments. Since the conventional cooling fans are not provided with any protective structure against the severe environments, they are actually not suitable for use in the environments constantly invaded by foreign matters.
It is known a rotor for the conventional cooling fan includes, from outside to inside, a plurality of blades, a housing, and a magnetic element. In consideration of good magnetic induction, a space left between the rotor and a motor of the cooling fan should not be purposefully increased. The space is considerably small and narrow compared to the size of the whole cooling fan. Generally, the magnetic element is made of a rubber material and the blades are made of a plastic material; therefore, both of them are relatively soft parts. Since the conventional cooling fan is not provided with any mechanism for removing the invaded tiny foreign matters, such as impurities and crystals, from the small and narrow space between the rotor and the motor of the fan, some abrasive impurities and external foreign matters tend to accumulate in the small space to constantly frictionally contact with the rotor, causing quick wear and damage of the rubber-made magnetic element and plastic-made blades. In worse conditions, the fan will become stuck or fail very soon and accordingly has largely shortened service life.
In addition, the housing and the magnetic element for the conventional fan rotor have relatively high surface roughness due to their materials, and therefore have higher resistance to the air flows produced by the fan when they flow through the housing and the magnetic element, which in turn causes unstable flow field around the fan.
In brief, the conventional fan rotor has the following disadvantages: (1) causing shortened fan service life; and (2) having higher resistance to air flows and therefore resulting in unstable flow field around the fan.